I keep getting this error when opening any repository after updating to the newest version of Sourcetree (on Win 10):

I am even getting the error after cloning an empty Bitbucket repository. It's exactly the same error for every repository I open. I can close the error message but not push or pull any changes with a similar error message:
warning: unable to access '.': Permission denied
fatal: cannot use . as an exclude file
Can somebody please help with this?
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Addition:
I tried uninstalling and deleting all preferences, then reinstalling the older version 2.5.5.0 (which funny enough is the newest version you can download from atlassian.com). But the error still appears.